As soon as one of mine starts sitting, they all pile in. It can only be a survival technique, no?
A way of spreading their eggs to more than one nest and therefore their chances of survival, as in: 'Don't put all your eggs in one basket'?
I think it's more of the other hens thinking the broody should just move on and give them their turn at the nest. In the beginning I made sure I had plenty of nesting and they all fought over the same nest! It's still the same, lots of room for eggs and they all fight over one spot.